1. Technical Field of the Invention
The present invention relates generally to an optical fiber fluorosensor and more particularly an improved fluorosensor capable of determining the concentration of any chemical species.
2. Discussion of the Related Art
Absorption and emission of evanescent waves are well-known phenomena that have been theoretically and experimentally investigated and widely used for sensing purposes. For example, absorption of evanescent waves is used to determine the concentration of methane-gas with a tapered optical fiber. In this approach, a He-Ne laser excites bound modes in the fiber. The chemical species surrounding the tapered region of the fiber absorbs the evanescent wave associated with these modes at a specific wavelength. This absorption can be detected at the end of the fiber as a decrease in the output signal level and the concentration of the species inferred.
Using evanescent wave coupling, an optical fiber sensor has been developed with a fluorescent cladding to detect molecular oxygen. Evanescent waves are a factor whenever radiation is totally internally reflected between two dielectric media having different indices of refraction. Although most of the incident power is reflected, part of the radiation, termed the evanescent component of the field, penetrates a very thin layer of the dielectric having the lower index of refraction. Specifically, an optical fiber is clad during manufacture with a polymer such as polydimethyl siloxane which has a fluorescent dye dissolved therein. The dye itself is sensitive to the presence of molecular oxygen. The fluorescent cladding was excited via evanescent waves upon side-illumination at a wavelength within the excitation range of the dye. As before, some light was trapped in the core by evanescent coupling. In a similar sensor, an oxygen sensitive fluorescent coating was applied to a fiber having a fluorescent core. The light emitted by the dye in the cladding excited the fluorescent sources in the core. The result was a 100-fold increase in the efficiency of the sensor when compared with the previous one. The fluorescence intensity is a measure of the partial pressure of molecular oxygen.
These conventional fluorosensors are characterized by the weakly guiding approximation, which is also used to model communications fibers. This approximation is based on the assumption that small differences of approximately 0.01 or less between the respective index of refraction for a fiber core and a cladding are desired. This small difference in effect confines the optical model to one index of refraction, whereas a true optical fiber has two indices of refraction.
In addition, geometric optics theory has been employed to determine chemical concentration. However, this method cannot be applied to fibers having a few modes. Also, this method concentrates solely on the corpuscular nature of light whereas the injection of light from cladding sources is properly characterized as a wave phenomenon.